The present invention relates to a method of and an apparatus for starting a gas turbine plant, particularly a gas turbine plant incorporating at least one two-stage combustion type combustor for suppressing the generation of NOx.
In recent years, plans have been proposed in Japan for development of a complex cycle power-generating plant in which a gas turbine plant and a steam turbine plant are combined, in order to meet the demands for the saving of energy and the preservation of natural resources. At the same time, in Japan, there are strict requirements of regulations for controlling exhaust emissions from gas turbines, in order to prevent pollution. Unfortunately, existing gas turbine combustors cannot clear the levels required by the regulations, particularly in connection with nitrogen oxides (NOx), and this is the reason why the development of combustors which generate less NOx is becoming a matter of urgency.
An effective method of suppressing the generation of NOx in gas turbine combustors is to supply the fuel to a plurality of portions of the combustor so that combustion is completed in a plurality of stages according to the load on a gas turbine. The most practical form of this type of combustion system is the so-called two-stage combustion system which has been already put to practical use.
The two-stage combustion type combustor has a primary combustion chamber for the combustion of a primary fuel and a secondary combustion chamber downstream from the primary combustion chamber and adapted for the combustion of a secondary fuel at a comparatively high load range. The ignition of the secondary fuel in the secondary combustion chamber is effected by the flame generated in the primary combustion chamber, so that the fuel-air ratio of the mixture to be burnt in the secondary combustion chamber can be large, i.e., the mixture can be lean, which, as is well known to those skilled in the art, suppresses the generation of NOx during the combustion.
In general, the rate of fuel supply to a gas turbine plant changes substantially in proportion to the load, whereas the rate of supply of air from the compressor is substantially constant, because the speed of the gas turbine is almost constant. This inevitably causes the fuel-air ratio to be changed. Therefore, a problem is encountered in that the mixture of an fuelair ratio optimized for a specific load range is often too rich or too lean in other load ranges, resulting in an increased emission of noxious components such as NOx, CO and HC.
To obviate this problem, as shown in ASME paper 84-GT-44 (Oct. 1, 1984), it has been proposed to provide a gas turbine with a by-pass passage through which a part of the compressed air is introduced directly into the downstream side of the combustor, together with a suitable means for controlling the flow rate of the air through this by-pass passage in accordance with the load of the gas turbine.
In this known arrangement, the variable area of the by-pass passage has to be large enough to provide the required secondary air flow rate. This, however, is generally difficult to attain, owing to restrictions on the structure of a gas turbine. In consequence, the gas turbine operation with both the primary and secondary combustions is possible only in comparatively heavy load range. During steady operation of the gas turbine, therefore, the switching of operation from a primary combustion mode to a primary and secondary combustion mode takes place at a comparatively high level of load, accompanied by abnormal combustion pulsation.
For the purpose of suppressing the combustion pulsation, the switching from the primary combustion mode to the primary and secondary combustion mode is preferably conducted while the load level is comparatively low.